The images are "horrendous" and, if authentic, are a serious breach of international law.

The United Nations’s own expert called for an investigation into footage broadcast by this programme.

But there are accusations that the organisation failed to speak out about alleged atrocities committed in the dying days of Sri Lanka's war against the Tamil Tigers.

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Philip Alston, who is the UN's special rapporteur on extrajudicial, summary or arbitrary executions, explained whether he thought the pictures were authentic.

He said: "This videotape seems to have most of the characteristics of a genuine article and that in itself is sufficient to impose an obligation upon a government to undertake a sustained, effective, impartial investigation to ascertain the truth.

"I think the United Nations is in a difficult situation in the sense that it's really a grouping of states and the government of Sri Lanka has been very effective in terms of garnering the support of a large number of states and that does seem to have made the UN rather reluctant to speak out on these issues.

"My own role is not that of a UN official. I am a so-called independent expert, appointed as a UN special rapporteur. I report to the UN human rights council.

"I am therefore able to say very clearly that these images are gravely disturbing, that they raise prima facie concerns about significant extra-judicial executions and a full-scale investigation should be undertaken.

"I would like to see a more active United Nations response to this issue. I think it's important to try to remove it from politics. I think the UN has to insist in relation to these sorts of issues that it is not putting the Sri Lankan government in the dock as it were.

"The UN's objective should be to ascertain the facts, to verify whether killings of this nature did take place either on an isolated basis or a systematic basis and thus to provide the foundations for an ongoing dialogue with the government of Sri Lanka.

"It shouldn't be seen as a one-off issue of 'do you condemn or do you support the government?'. The focus must be on the alleged killings and getting to the truth of those and then working out what should be done by all of the parties."